1. Field of the Invention
The present invention relates generally to optical communication systems and, more particularly, to a tunable optical heterodyne receiver circuit monolithically integrated onto a single semiconductor chip that recovers the entire information signal by summation and achieves cancellation of any excess intensity noise that may be generated by the local oscillator laser.
2. The Prior Art
Communication systems transmitting voice, video and data, are in the process of being upgraded from radio-frequency (RF) carriers to optical carriers. The development of lasers, and especially semiconductor lasers, has made it possible to transmit information by modulating one of the parameters of the optical field, typically its intensity. It is also possible to modulate the frequency or the phase of the lightwave. Increasingly, optical fibers are used as waveguides for transmitting the signals from a point of generation to the premises of a user. Low signal attenuation and large bandwidths are the primary characteristics that make optical fibers so attractive for telecommunications applications.
Optical fiber waveguides presently in use have bandwidths that generally exceed 20 THz. In principle, many separate signals may be broadcast in the same fiber system, each one transmitted at a slightly different frequency of the optical carrier, a technique known as Frequency Division Multiplexing (FDM). However, at present, upon detection of the optical field, the transmitted information signals are downconverted by an electronic circuit to basebands having substantially smaller bandwidths, that is about 1 GHz or less. There is no way to differentiate multiple channels. Consequently, the full capabilities of the optical fiber transmission waveguides remain severely under-utilized. An optical fiber has the potential to carry simultaneously more than 1000 separate signal channels, each requiring a bandwidth on the order of 1 GHz. Means must be provided, however, for demultiplexing all of these channels.
For the past five decades, almost all communication systems operations in the radio-frequency (RF) segment of the spectrum have relied upon heterodyne detection because of the channel selectivity, along with heightened sensitivity, afforded thereby. Heterodyne reception is based on combining the incoming radio-frequency signal with a locally generated RF signal of slightly different frequency. The information signal is down-converted to an intermediate Frequency (IF), which is the difference between the original signal carrier frequency and the local oscillator frequency. By tuning the local oscillator frequency, the user may convert any broadcast signal to the same IF; all other channels are rejected by a single electrical filter which is set to pass only IF. The IF is subsequently detected, amplified, and made available to the user. Thus choice of channel is available in RF systems. Presently however, most if not all commercial optical communication systems rely on simple direct detection of the optical carrier. Direct detection makes no use of either the wavelength or the phase of the light wave but only counts the modulation of the intensity of the optical signal. Optical heterodyne detection will play an ever more important role in future communication systems. Heterodyne detection, also known as coherent detection, as opposed to direct detection, is therefore gaining importance in long-distance optical communication systems, such as transcontinental telephone systems. The changeover to optical heterodyne communication systems has, however, been retarded by the physical complexity of the receiver designs. In order to utilize to the fullest the recent advances in system capabilities offered by optical heterodyne techniques, it is necessary, amongst other requirements, that the receiver components be monolithically integrated onto a single chip.
In coherent detection, a weak received signal is mixed with a strong local oscillator (LO) wave at a close enough frequency, resulting in an effective signal gain through coherent phase interference. The gain is proportional to the local oscillator amplitude. The recovered electrical signal power at IF is proportional to the product of the amplitude of the optical information signal and the amplitude of the local laser field. In direct detection, the recovered electrical signal power is proportional to the square of the amplitude of the incoming optical signal. If the amplitude of the local oscillator exceeds the amplitude of the incoming optical signal, which is the usual case, then in the heterodyne system, gain is proportional to their ratio.
Signal-to-noise ratio is a vital parameter in a communications system. In a heterodyne receiver, recovered IF signal power is proportional to the product of the optical signal power and the local oscillator power. Obstructing noise is proportional to the sum of the two optical power levels plus the noise power generated in subsequent electronic amplifiers. If the local oscillator power is made sufficiently large, all factors cancel except the optical signal power giving the ultimate receiver sensitivity of one photon per bit to get a signal-to-noise ratio of one. There are some inherent problems, however. For any semiconductor laser there is an additional noise source, called excess intensity noise. This excess noise will not cancel from the signal-to-noise ratio relationship. It severely degrades the performance of a heterodyne receiver. As the local oscillator signal power is increased, the measured shot noise in the detector and the noise from the laser often exceed the signal gain. A balanced detector configuration, in which two identical detectors are connected together in electrical series, allows the suppression of the local oscillator laser intensity noise.
A properly functioning optical heterodyne receiver requires three basic components: a set of parallel waveguides, a tunable laser, and a means of detecting the interference pattern generated by the coupling of the two optical waves in the adjacent waveguides. Since there are two optical waves present in two waveguides, there will always be two output signals, preferably of equal amplitude, but also 180.degree. out of phase. Thus the receiver should provide two detectors. Due to considerations of collecting all of the inherent signal power while suppressing excess noise in the receiver, these two detectors require a particular placement. No presently known design of an integrated optical heterodyne receiver chip advantageously places the two detectors thereon.
Early versions of balanced optical heterodyne receivers were based on the use of hybrid RF junctions or transformers to allow the signals from the two detectors to be subtracted. There are advantages inherent in directly connecting the two photodiodes to a common FET amplifier. First, thermal noise generation can be kept low because the input impedance can be made high while the capacitance can be minimized. Second, because the detectors are connected directly in series electrically and both are subjected to a proper level of reverse bias, the DC component of the photocurrent, due to the local oscillator, flows directly to ground and not into the amplifier input. DC saturation of the amplifier input is thereby avoided. Most important, monolithic intergation becomes possible. Many dual detector schemes have recently been presented, but they always fabricate the two detectors adjacent one another on the substrate, basically in parallel electrically. Connecting the detectors in electrical series therefore is basically not possible, unless an insulating substrate is used, which makes it extremely difficult to form electrical contacts for the tunable laser. On a conducting substrate, when the detectors have been situated in electrical parallel, it has been necessary to perform signal processing external to the chip with a microwave phase shifter/combiner in order to achieve signal addition and noise cancellation. Such external processing structure effectively doubles the thermal noise of the front end of the receiver, however.